Buying a Violin Rosin
Violin Rosin is one of the essential items to use in order to produce sound with the bow. When you buy a new bow and try to play with it you will soon realize that the hair of the bow is slighting on the strings without yet producing (if any at all) the expected sound. The reason is that the bow has no rosin applied on it. So the main function on rosin is to make the bow-hair stick better on the strings.
Violin rosin is a hardened tree sap that you apply to your violin bow before every other practice in order to give it the necessary grip when drawing the bow across the strings on your violin. Without it, the bow simply slides across the string without grabbing it, producing very little sound.
You can find rosins in various colors and shapes. Violinists are using darker or lighter rosins depending on the environmental/climate conditions and their personal preferences. Generally speaking lighter rosin help to create a smoother and softer sound while the dark rosins (harder and stickier) tend to make the sound bigger and sharp).
Some things to consider before buying a rosin
In general humidity makes the rosin stickier than usual so it is preferable to use lighter rosins to smoother your sound and the opposite for dry climates. Extremely humid conditions however (like having to play in open air next to the see in Cyprus for example:) ) may wash out the rosin from the bow. In that case it is recommended to use regularly the dark rosin
Over time as you play, rosin dust accumulates on your violin that you will periodically need to wipe away with a soft cloth (there are special violin polishing cloths you can get specifically for this purpose).Darker violin rosin tends to produce more dust (i.e. you must clean your violin more frequently) than light rosins, while light rosins can take more time to wipe away when cleaning your violin. The difference is subtle, but if you are sensitive to dust you may want to select a lighter rosin to cut down on the amount of dust in the air.In general good quality rosins produce less dust than others.
For people that allergic to violin rosin there are some special hypoallergic rosins that reduce the discomfort that the dust of the rosin causes.
Rosins are coming in different package — shapes. Always note you need to consider this factor with care as the shape of the rosin will affect the way you will apply the rosin on the bow, the extent you will make use of it and the endurance. Children (beginner violinists) are usually given rosins with a square or rectangle shape with the sides being protected with wood and plastic. The reason is to facilitate the use of it (as the children are not yet coordinated enough to keep the bow on the rosin while pulling it along the bow) and to protect it as rosin is extremely fragile. However, unless you have coordination issues, I strongly recommend against this type of packaging, opting instead for a round or square shape.
The round rosins are in general easier in use more economical as you can make use of all the available rosin by turning in cyclical motion while applying it on the bow.
Furthermore, I recommend getting rosin in robust packaging that can withstand shocks of traveling in your violin case, as it will be less likely to fracture prematurely, and will therefore last longer.
Here you can find a big variety of rosins.